Safety guards or barriers which shield the moving parts of a machine in order to prevent accidental injury are well known. A problem with safety guards is that they restrict essential access to the workstation of the machine such as is necessary for an operator to properly and efficiently position a workpiece in the workstation, thereby frustrating the operator and reducing production rates. For a non-continuous duty cycle machine of the type which is cycled only upon receiving a signal initiated by the operator, it is advantageous to have a movable safety guard which is open during periods between machine cycles to allow better access to the workstation, but which closes when the operator cycles the machine. Still, since the movable safety guard must close before a critical machine operation can occur, machine cycle time is increased and production rates are reduced. Further, since some operators want to hold on to a part of the workpiece in the vicinity of the safety guard, there is a tendency for operators to defeat the safety guard such as by binding it in the open position. Even so, continuing efforts to improve worker safety are resulting in ever more restrictive safety guards.
U.S. Pat. No. 4,060,160 to Lieber discloses a safety guard which is mounted to move with the ram of a machine. A safety interlock ensures that if a foreign object is encountered by the guard, machine operation will be terminated. The interlock utilizes the safety guard as a workpiece sensor and relies on a material property of the workpiece not normally exhibited by a body part, such as electrical conductivity. If the safety guard encounters an object without concurrently sensing the expected material property, power to the ram is cut off. There are a number of problems with this arrangement. A specific material property of the workpiece must be selected to be sensed, thereby requiring a sensor which is dedicated to the selected material property. Also, the interlock may need to be customized with different sensors for different material properties of different workpieces. Further, some workpieces may not readily lend themselves to being sensed through the safety guard. For example, an insulated electrical conductor which is to have a terminal applied to a crimped end thereof is not readily differentiated from a foreign object based on a material property. There is a need for a movable safety guard with a simple interlock system to ensure that the guard has closed prior to an operation being performed on a workpiece.